The invention relates to an electric discharge lamp comprising:
a light-transmissive ceramic lamp vessel;
a first and a second current conductor entering the lamp vessel and each supporting an electrode in the lamp vessel;
a ceramic sealing compound sealing the lamp vessel around the current conductors in a gastight manner;
an ionizable filling comprising a rare gas and metal halide in the lamp vessel, at least the first current conductor within the lamp vessel having a first halide-resistant part and a second part extending from the ceramic sealing compound to the exterior of the lamp vessel.
Such an electric lamp is known from U.S. Pat. No. 5,424,609.
The current conductors of such a lamp must have a linear coefficient of thermal expansion which corresponds to that of the lamp vessel in order to prevent leakage of the lamp. Leakage may even occur in the manufacture of the lamp when the lamp cools down after the ceramic sealing compound has been provided at a relatively high temperature. At a too small coefficient of expansion of the current conductor, the lamp vessel shrinks to a stronger extent and it may crack or even break. At a too large coefficient of expansion, leakage may occur around the current conductors.
However, the current conductors must also be resistant to the ionizable filling of the lamp, particularly to halide, at least in so far as they are in contact therewith: they should at least not substantially be attacked by or react with halide or halogen formed therefrom. A low resistance may not only result in damage and destruction of the current conductor but also in a loss of halide in the filling and in a color change of the light generated by the lamp. Moreover, the current conductors must withstand the thermal manufacturing and operating conditions of the lamp and, to inhibit electrical losses, they should be good conductors.
Since the requirements imposed on expansion and chemical resistance are often not combined in one material, at least the first current conductor of the known lamp within the lamp vessel has a first halide-resistant part having a different expansion than the lamp vessel, and a second part which extends from the seal and is not halide-resistant but has a corresponding expansion. This part often consists of niobium, tantalum or an alloy thereof, metals which, due to their oxidation sensitivity at higher temperatures, should be screened from air by using an outer envelope for the lamp.
If the lamp vessel is relatively narrow and elongate, and if it has a vertical operating position, the halide and the halogen formed therefrom are particularly present in the lower portion of the lamp vessel. It is then sufficient when only the first current conductor has a first halide-resistant portion and is present in the lower part of the lamp vessel. However, the lamp can then not be operated upside down, horizontally or obliquely. However, for obtaining a universal operating position, the lamp can be given a second current conductor corresponding to the first.
The first part of the current conductors of the known lamp has at least at its surface tungsten, molybdenum or molybdenum disilicide. The first part may be alternatively a solid rod of the materials described.
It is a drawback of the known lamp that leakage does occur if the ceramic sealing compound extends as far as the first part and also connects this part to the lamp vessel. Nevertheless, it may be necessary to surround the second part of the current conductors within the lamp vessel entirely with the ceramic sealing compound so as to protect it from a halide attack. It has then proved to be difficult to provide the ceramic sealing compound in such an amount that the material at least substantially surrounds the second part but does not directly connect the first part to the lamp vessel.
It is an object of the invention to provide an electric lamp having a construction which is easy to manufacture and obviates the risk of leakage owing to ceramic sealing compound which also directly connects the first part of a current conductor to the lamp vessel.
According to the invention, the first part of the first current conductor consists essentially of an intermetallic compound material chosen from tungsten silicide, pentatungsten trisilicide, molybdenum aluminide, molybdenum boride, pentamolybdenum trisilicide and combinations of at least two of these materials.
In a preferred embodiment, the second current conductor 9150 has such a first and a second part. This embodiment simplifies the manufacture of the lamp because the same components are used for both current conductors. The lamp can then be operated in an arbitrary position, while halide attack and the risk of leakage are inhibited.
It was found that tungsten silicide in the form of WSi2 and in the form of W5Si3, molybdenum aluminide, Mo3Al, molybdenum boride, MoB, and pentamolybdenum trisilicide, Mo5Si3, have a linear coefficient of thermal expansion which corresponds to that of the lamp vessel. These intermetallic compounds are thermally and chemically stable in the circumstances of manufacturing and operating the lamp. This is in contrast to the molybdenum disilicide mentioned in U.S. Pat. No. 5,424,609, which decomposes when used as a material of the first part of the current conductor(s) upon welding to the electrode and to the second part of the current conductor(s). The materials, particularly Mo3Al and notably WSi2 can easily be processed as well as W5Si3 and Mo5Si3.
The intermetallic compounds may be used in the lamp as sintered bodies or as wires or rods drawn from sintered bodies Although this is generally not necessary, a small volume, for example, several tenths of percents to several percents of metal having a relatively low linear coefficient of thermal expansion such as tungsten or molybdenum may be added to the intermetallic compound so as to give the expansion an even greater conformity with that of the lamp vessel.
Due to the favorable coefficient of expansion, the second part of a current conductor may consist of the same material as the first part and this current conductor may even be one integral body. This saves a welding operation.
It is not objectionable if the current conductors do not have a second part of hydrogen-transmissive material because the presence of water from which hydrogen is produced in the lamp can be substantially prevented by careful manufacture. Moreover, the ceramic lamp vessel itself is hydrogen-permeable at the relatively high operating temperatures and the lamp may be operated, for example initially, at a power supply which can obviate an increased ignition voltage owing to the presence of hydrogen.
An important advantage of current conductors with a second part extending beyond the lamp vessel and made of the same material as for the first portion is that the material is also resistant to oxygen at a higher temperature so that the lamp can be operated directly in air and does not need an outer envelope which is sealed in a gastight manner.
It is favorable if the electric lamp has a lamp vessel with narrow end parts in which a respective current conductor is enclosed, the end parts having a free end where the lamp vessel is sealed by the ceramic sealing compound. This embodiment has the advantage that the ceramic sealing compound is relatively far remote from the electrodes and thus has a relatively low temperature, while yet preventing that the lamp vessel behind the electrodes has a relatively large volume of low temperature where halide could condensate and could thus be withdrawn from the discharge. The volume of the end parts is small and is also sufficiently heated due to the passage of current through the current conductors so as to prevent accumulation of halide.
The ionizable filling may not only comprise a rare gas as an ignition gas such as, for example argon, but also one or more halides, for example iodides such as, for example, a mixture of iodides of Na, Tl and Dy, possibly with Ho and Tm, or a mixture of iodides of, for example, Na, Tl, Ca and Ho so as to radiate light at a color temperature of 3000 K., or a mixture of iodides of, for example, Na, Tl, Ca, Ce, Dy, Ho and Tm so as to generate light at a temperature of 4000 K.
The lamp vessel may consist of mono or polycrystalline material such as aluminium oxide or sapphire.
The ceramic sealing compound may be, for example, a mixture of aluminum oxide, silicon oxide or dysprosium oxide or magnesium oxide.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.